Microsoft is writing down $6.2 billion on its 2007 purchase of aQuantive, an online ad serving system. According to the Guardian this places the software giant at risk of an accounting-based quarterly loss (when its sales and profitability had aimed it at a profit of around $5 billion), and demonstrates the catastrophic fail of one of Microsoft’s biggest ever purchases, and came a the same time as Google spent $3.1 billion to buy DoubleClick. The online advertising game is notoriously tricky to get right, and has recently resulted in technical difficulties even for Facebook–which is used regularly by nearly one in seven people on the planet.